Figure 1 – Norman Rockwell – cover of the May 1, 1920 issue of The Saturday Evening Post, showing a Ouija board in use. Image in the public domain in the United States because of its age.
Today is March 27 and the last clear night that I had with my telescope was March 11 – getting rather annoying! Following up on my blog yesterday about the Bortle Scale, I’d like to ask today, how can one predict “Clear Skies?” I have used a number of proverbial apps to try to accomplish this.
The first was the ten day forecast from The Weather Channel with its hourly prediction. I find that it works pretty well, but doesn’t really give you the insight that you need at the level that you need it. I think the obvious reason is that they’re goal is not astronomical.
Next I tried SkyLive. This I found has beautiful graphics but wasn’t very useful for me,
Then I tried Astrospheric which actually was of quite reasonable usefulness.
Then I was driven to pay the reasonable price for GoodtoStargaze. This is my GOTO app and the one I recommend. I use it all the time, and augmented with The Weather Channel and sometimes Astrospheric, I find that I can usually do quite well at predicting the sky conditions. This is not to say that I have not had the experience of going out on a “clear” night only to be confronted with a mass of cumulus clouds. As a physicist I respect the fact that predicting massive weather systems with what I am told are unstable differential equations is iffy at best and you are easily off by ten miles or so.
Originally I would record the sunset or sunrise and moon conditions and leave it at that. Slowly my observing notebook is morphing into true scientific notebook, and as far as sky conditions beyond Bortle Number, I record a lot of parameters with the hope/plan of someday using my imaging successes and failures predictively. I may even create an AI app to scrape my notebooks for best conditions for what.
The parameters that I record are:
Sunset/sunrise – since you usually need at least 45 min into darkness befor you can align scopes |
Moonrise – the moon is a major and variable source of light pollution |
Moonset – when will this annoyance go away. TC is always saying to me, “Just look at how beautiful the full moon is tonight!” And I’m like, “eh……” |
Moon Phase – note that percent lunar illumination is a bit different and you can use that instead |
Temperature range – obvious point too cold, even with the observacar and I’m not going out |
Windchill – this for the same reason |
Dew point – is kind of a predictor when you need your lens heater |
Relative Humidity – this for the same reason |
Wind conditions – this comes in two flavors steady wind. I set a limit of about 25mph and gusts 30 mph. Don’t want my telescope falling over or vibrating too much for imaging |
Cloud cover – I really like this below 25 % |
Seeing – this is the discernable star resolution with the naked eye. As I’ve gotten older my naked eye has gotten more and more modest. But the basic point here is that the smaller this number is the finer detail you can see. |
Transparency – this is a tough one but basically is the magnitude loss of a star going through the air mass of the atmosphere |
Visibility – this is the usual how far you can see. I number of 10 miles or better is what you are looking for. |
Light pollution – this is our favorite the old Bortle Number, which usually doesn’t change much day-to-day |
Notes – whatever else you want to record. |
Every entry in my observing notebook begins with these numbers.
A lot of times when my apps are predicting clear skies and cumulus clouds or worse rains abound, I start to wonder if there isn’t a device that would be at least as good at predicting sky conditions. Such a device is shown in Figure 1 and is called a Ouija Board!